Add authentication: admin/agent bearer tokens for the API, login for the dashboard

control-api: every route now carries a mandatory access level (admin / agent /
open) in a route table. All /api/v1/admin/* require the admin token; the
write calls of validator-agent and prober (self-check, events, results,
complete) require a separate static agent token; register, heartbeat and
fetching the assignment stay open. Tokens come from env vars, are compared in
constant time and never logged. An empty token leaves that level open with a
startup warning (backward compatible).

validator-agent / prober: apiclient sends the agent token only to control-api.

admin-dashboard: login/password (from env) with a stateless HMAC session
cookie, Origin-based CSRF check, per-IP brute-force throttle, HX-Redirect for
htmx polls, logout in the sidebar; the dashboard calls control-api with the
admin token. Login page layout fixed after review.

Also: env plumbing in docker-compose/rxprod-compose/systemd/config examples,
e2e script with token assertions, tests, docs (API, SETUP, USAGE, DASHBOARD,
README), plan and review under docs/changes/, bin/ rebuilt with new
SHA256SUMS.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/dashboard/client.go

@@ -39,6 +39,8 @@ func (e *apiErr) Error() string {
 type client struct {
 	baseURL string
 	http    *http.Client
+	// token, when non-empty, is sent to control-api as a Bearer credential.
+	token string
 }
 
 func newClient(baseURL string, timeout time.Duration) *client {
@@ -61,6 +63,9 @@ func (c *client) do(ctx context.Context, method, path string, body, out interfac
 	if body != nil {
 		req.Header.Set("Content-Type", "application/json")
 	}
+	if c.token != "" {
+		req.Header.Set("Authorization", "Bearer "+c.token)
+	}
 
 	resp, err := c.http.Do(req)
 	if err != nil {